134 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
adopted, and an arbitrary scale was taken, in which the inch meant that 
it magnified a certain amount with a certain tube-leugth. But nobody 
knew where the tube-length was measured from. An American 
gentleman the other day published a paper in which he tabulated all the 
various tube-lengths as measured by various English and Continental 
manufacturers, and they varied most enormously. But, even supposing 
a definite tube-length to be taken, the difficulty still remained, because 
in order to get a magnifying power which should be in any way 
consistent with changed eye-pieces and object-glasses, it was necessary 
that low-power object-glasses should be mounted in enormously long 
tubes and high powers in very short tubes. When using a low power 
you then had to have it a long way from the object to begin with, and 
that difficulty was increased by mounting it in a tube 3 in. or 4 in. long, 
which in the case of a 5 in. would be simply preposterous. Then, 
again, supposing that, for the sake of scientific accuracy, such a plan 
were adopted, — what could be done in the use of a binocular Microscope ? 
With the binocular Microscope it was essential to have the object-glass 
as near as possible to the binocular prism, whereas if lenses were 
mounted on the principle suggested, the low powers, which are the very 
powers used for binocular work, ought to be mounted a long way away 
from the tube of the Microscope. Until, however, this plan was adopted, 
no really true method of magnification could possibly be established. 
As a matter of fact opticians at present were making their low-power 
lenses very much higher in power than they ought to be in order to 
obtain this standard magnifying power which was adopted as an arbitrary 
scale. For instance, a modern 4-in. objective was nothing like 4 in.; it 
was nearer 3 in., because its nodal point was too far up the Micro- 
scope. It was put up to a much higher power to produce the same 
magnifying power in connection with the same eye-piece. This plan got 
over the difficulty tolerably, but when you changed the eye-piece you 
began to find that although with one particular eye-piece the magnifying 
scale was tolerably constant — and could be made absolutely constant — 
when you changed the eye-piece it did not affect different powers in the 
proper ratio, and the scale was thrown out. Mr. Whipple said the 
nodal points were not of so much importance in jihotographic and other 
optical lenses, and he was quite right in saying that their position was 
not of nearly so much importance, but it was extremely important some 
means should be adopted which eliminated the distances between the 
nodal points in measuring the focal length of optical instruments. 
Prof. Thompson, in reply, said Zeiss’s way of getting over the 
difficulty about tube-length was not quite as Mr. Beck had stated, 
because Zeiss distinctly attempted to regulate the depth to which the 
eye-pieces were to be plunged down, so that they should not overlap in 
the same way as in the old arbitrary scale. Whether he was successful 
in carrying that out with very low powers he could not say. In reply 
to Mr. Dallmeyer’s question, he said that the present instrument would 
not measure lenses of more than 6 in. focal length. Mr. Whipple 
seemed to give no hope that the practical user of lenses would be 
content to change the misleading term astigmatism ; but what were 
you to do if you came across a lens which had two defects, and both 
were called astigmatism ? If they meant different things you must give 
